1. Summary of the Invention
2. Field of the Invention
The system disclosed herein relates generally to the field of photovoltaics, and more specifically to fabrication of unique, thin-film, high-output, flexible chalcopyrite copper indium diselenide (CIGS) photovoltaic modules.
3. Background of the Invention
Looking briefly at the background for the field of the present invention, the field of photovoltaics generally relates to the development of multi-layer materials that convert sunlight directly into DC electrical power. In the United States, photovoltaic (PV) devices are popularly known as solar cells--which are typically configured as a cooperating sandwich of p- and n-type semiconductors, wherein the n-type semiconductor material (on one "side" of the sandwich) exhibits an excess of electrons, and the p-type semiconductor material (on the other "side" of the sandwich) exhibits an excess of holes. Such a structure, when appropriately located electrical contacts are included, forms a working PV cell. Sunlight incident on PV cells is absorbed in the p-type semiconductor creating electron/hole pairs. By way of a natural internal electric field created by sandwiching p- and n-type semiconductors, electrons created in the p-type material flow to the n-type material where they are collected, resulting in a DC current flow between the opposite sides of the structure when the same is employed within an appropriate, closed electrical circuit. As a standalone device, conventional solar cells do not have a sufficient voltage required to power most applications. As a result, conventional solar cells are arranged into PV modules by connecting the front of one cell to the back of another, thereby adding the voltages of the individual cells together. Typically a large number of cells, on the order to 36 to 50 are required to be connected in series to achieve a nominal usable voltage of 12 to 18 V.
Commercial use and interest in thin film photovoltaics (PV) has expanded dramatically in the last five years, however commercial wide-scale use for bulk power generation remains limited primarily by two factors: (1) performance and (2) cost. Over the past few years dramatic improvements in PV module performance have been achieved in both crystalline silicon and thin film photovoltaics. Laboratory scale efficiency of crystalline silicon is approaching 20%. Modules ranging from 10 to 14% are commercially available from several vendors. Similarly, laboratory scale efficiencies of well above 10% have been achieved with thin film PV devices of copper indium diselenide, cadmium telluride, and amorphous silicon, most notably the record efficiency for CIGS is now approaching 19%. Additionally several companies have achieved thin film large area module efficiencies ranging from 8 to 12%. As a result of these recent advances, performance no longer seems to be the key limiting factor, leaving cost as the primary factor preventing wide-scale commercial use of PV modules for electricity generation.
Thin film based photovoltaics, namely amorphous silicon, cadmium telluride, and copper indium diselenide, offer improved cost by employing deposition techniques widely used in the thin film industry for protective, decorative, and functional coatings. Common examples of low cost commercial thin film products include water permeable coatings on polymer-based food packaging, decorative coatings on architectural glass, low emissivity thermal control coatings on residential and commercial glass, and scratch and anti-reflective coatings on eyewear.
Copper indium gallium diselenide (CIGS) has demonstrated the greatest potential for high performance, low cost thin film PV products. Of the thin-film PV, CIGS has achieved the highest laboratory efficiency (18.8% by NREL), is stable, has low toxicity, and is truly thin film (requiring less than two microns layer thickness). The above factors all lead to a high potential for CIGS to be economically manufactured on a large scale, thereby penetrating bulk power generation markets.
Thus far, however, and remarking here briefly on a de facto prior art shortcoming that relates to attractive and encouraging use of CIGS/CIS materials, the majority of CIGS/CIS research and development has been concentrated to date on the deposition of thin-film layers of these materials on rigid glass substrates. Key attributes of glass are the cost of the base raw material, good coefficient of thermal expansion match with the CIGS device layers, and vacuum deposition system designs exist. Such rigid substrates are not ideal in many circumstances, and do not lend themselves readily to large-volume, high-yield, commercially manufacturing of multi-layer functional thin film materials such as photovoltaics.
Major contributing factors of rigid substrate processing on cost are:
1. Large scale processing of glass requires substantial floor space for 2-dimensional (x, y) processing equipment and material storage, PA1 2. Large sections of plate glass require specialized heavy duty handling equipment, PA1 3. Continuous, automated in-line processing of a multi-layer structure (4-layers for CIGS), requires several additional chambers to isolate the layer-specific processing environment, a feature not encountered in most glass processing industries, PA1 4. Heating and cooling rates between processing zones is slow and must be carefully controlled to avoid fracturing the glass, PA1 5. Even with careful control of process heat and cooling times, glass fracturing often occurs and has a tremendous impact on yield, PA1 6. Volume and weight of the product impact shipping cost to the use site, substantially increasing cost, PA1 7. Installation, especially rooftop, requires additional labor (i.e., two-persons) to place the panels. PA1 1. Compact, less expensive vacuum systems, PA1 2. Complete process isolation based on batch processing of long rolls, PA1 3. Extreme tolerance to rapid heating and cooling as well as large thermal gradients, PA1 4. Robust structure is unlikely to fracture or fail during processing,
Roll-to-roll processing of thin flexible substrates addresses all the above issues. Key attributes of roll-to-roll processing of thin flexible substrates include:
In choosing flexible processing, a few issues are encountered that are addressed in the present system: (1) Molybdenum-deposition (Mo-deposition) approaches used for glass are unacceptable for flexible substrates, and (2) CIGS processing using traditional industrial approaches are not logical for continuous roll-to-roll processing.